Which are the verses in the Bhagavad Gita which clearly say caste is not by birth?

'Caste' is an English term. There is no exact equivalent in Sanskrit. Even 'Jati' and 'varna' do not mean caste. So naturally it is not found in the Bhagavad Gita!

cātur-varṇyaḿ mayā sṛṣṭaḿ guṇa-karma-vibhāgaśaḥ tasya kartāram api māḿ viddhy akartāram avyayam According to the three modes of material nature and the work associated with them, the four divisions of human society are created by Me. And although I am the creator of this system, you should know that I am yet the nondoer, being unchangeable. The Lord is the creator of everything. Everything is born of Him, everything is sustained by Him, and everything, after annihilation, rests in Him. He is therefore the creator of the four divisions of the social order, beginning with the intelligent class of men, technically called brahmanas due to their being situated in the mode of goodness. Next is the administrative class, technically called the kshatriyas due to their being situated in the mode of passion. The mercantile men, called the vaisyas, are situated in the mixed modes of passion and ignorance, and the sudras, or laborer class, are situated in the ignorant mode of material nature. In spite of His creating the four divisions of human society, Lord Krishna does not belong to any of these divisions, because He is not one of the conditioned souls, a section of whom form human society. Human society is similar to any other animal society, but to elevate men from the animal status, the above-mentioned divisions are created by the Lord for the systematic development of Krishna consciousness. The tendency of a particular man toward work is determined by the modes of material nature which he has acquired. Such symptoms of life, according to the different modes of material nature, are described in the Eighteenth Chapter of this book. A person in Krishna consciousness, however, is above even the brahmanas. Although brahmanas by quality are supposed to know about Brahman, the Supreme Absolute Truth, most of them approach only the impersonal Brahman manifestation of Lord Krishna. But a man who transcends the limited knowledge of a brahmana and reaches the knowledge of the Supreme Personality of Godhead, Lord Sri Krishna, becomes a person in Krishna consciousness -- or, in other words, a Vaishnava. Krishna consciousness includes knowledge of all different plenary expansions of Krishna, namely Rama, Nrisimha, Varaha, etc. And as Krishna is transcendental to this system of the four divisions of human society, a person in Krishna consciousness is also transcendental to all divisions of human society, whether we consider the divisions of community, nation or species.

1. In the Gita, Lord Krishna clearly said castes are born of deeds and qualities people nurture (contain) and it has nothing to do with birth in a particular community or religion or caste. The fourfold-caste has been created by Me according to the  differentiation  of GUNA and KARMA; though I am the author thereof know Me as non-doer and immutable. ------ Gita, Chapter-4, Verse-13. 2. Lord Krishna clearly said that to a learned man all are equal. Now you can judge yourself where one is standing on the truth plane. Sages look with an equal eye upon a BRAHMANA endowed with learning and humility, on a cow, on an elephant, and even on a dog and an outcaste. ---- Gita, Chapter-5, Verse-18.